1. Field of the Invention
The invention relates to a solenoid valve in which a spool provided in a valve hole is moved by electromagnetic force so that the state of communication among ports that open into the valve hole is adjusted.
2. Description of the Related Art
A solenoid valve as described in, for example, Japanese Patent Application Publication No. 2006-83879 (JP-A-2006-83879) is used for hydraulic control of, for example, an automatic transmission (AT) or a continuously variable transmission (CVT). This solenoid valve is formed of a solenoid portion and a spool control valve. The solenoid portion includes a plunger that is driven by electromagnetic force. The spool control valve includes a spool that is provided in a valve hole formed in a spool case and that moves within the valve hole in accordance with the movement of the plunger.
In the spool case, a supply port, an output port, and a drain port that open into the valve hole are formed so as to align in the direction in which the spool moves. The spool has a land that has an outer peripheral face that conforms to the inner peripheral face of the spool case. A flow-blocking portion having an overlap length corresponding to the movement amount of the spool is formed by the outer peripheral face of the land and the inner peripheral face of the spool case. By changing the overlap length of the flow-blocking portion formed between the ports, the state of communication between the ports is adjusted. As a result, the hydraulic pressure is controlled.
For example, in the non-energized state where electromagnetic force is not applied to the plunger, the spool is urged by a spring provided in the spool case and the land is placed at such a location that the flow-blocking portion is formed between the drain port and the output port. If the spool is at this location, communication between the output port and the drain port is blocked, communication between the output port and the supply port is provided, and the pressure in the supply port is introduced into the output port. On the other hand, when the plunger is driven by electromagnetic force, the spool is moved against the urging force of the spring and the land is placed at such a location that the flow-blocking portion is formed between the supply port and the output port. Thus, communication between the output port and the supply port is blocked, communication between the output port and the drain port is provided, and the pressure in the output port decreases.
In this solenoid valve, however, the hydraulic fluid leaks from a narrow gap between the inner peripheral face of the spool case and the outer peripheral face of the land, at the flow-blocking portion formed between the ports. As a result, the pressure in each port decreases in accordance with the amount of leaked hydraulic fluid. Therefore, in order to make up for a decrease in the pressure, the output from a pump that supplies the hydraulic fluid to the solenoid valve needs to be increased. In recent years, there has been a demand to reduce the flow amount of hydraulic fluid that is consumed (hereinafter, referred to as “consumption flow amount”) (i.e. flow amount of hydraulic fluid that is leaked) between the ports to suppress an increase in the output from the pump, in the view of energy saving. As one of the methods for reducing the consumption flow amount, increasing the overlap length of the flow-blocking portion formed between the ports may be employed. However, in order to increase the overlap length of flow-blocking portion, the movement amount (stroke amount) of the spool needs to be increased. This results in upsizing of the solenoid portion due to an increase in the amount of change in the load of the spring that urges the spool and a decrease in the solenoid attraction force. As a result, the accuracy of the hydraulic control may decrease.